The top quark mass has been measured using the template method in the t ¯t →lepton + jets channel based on data recorded in 2011 with the ATLAS detector at the LHC. The data were taken at a proton-proton centre-of-mass energy of √ s = 7 TeV and correspond to an integrated luminosity of 1.04 fb−1. The analyses in the e+jets and μ+jets decay channels yield consistent results. The top quark mass is measured to be mtop = 174.5±0.6stat ±2.3syst GeV.